The long range objective of this research is the definition of the structure and forces which bind two dissimilar ligands, cortisol and progesterone, to a common site on transcortin with an affinity (ca. 10 to the 8th power M-1) which far exceeds that demonstrated for most enzyme-substrate interactions. Experiments will follow the classical approaches which may be broadly divided into two general categories; One is the measurement of the effect of selective modification of the protein on binding and the other is affinity labeling. From the effect of pH on binding, it is likely that tyrosine or histidine (or both) are involved in the binding process. By the use of acetylimidazole of bromoacetate, respectively, these two residues can be modified and the effect on binding determined. Affinity labeling would utilize a 17- diazoketoacyloxprogesterone which will first bind and then react covalently in the binding site. After reaction cyanogen bromide treatment will split the protein into eight fragments; the one containing the covalently bound progesterone derivative will be analyzed in detail. The studies outlined above will be done on pure transcortin obtained from the serum of adults. In addition, transcortin will be isolated from cord blood, and its physical and chemical properties compared to that of transcortin from adults.